[petsc-users] Understanding index sets for PCGASM

Fri May 5 11:43:33 CDT 2023

  Added in barry/2023-05-04/add-pcgasm-set-subdomains see also https://gitlab.com/petsc/petsc/-/merge_requests/6419

  Barry


> On May 4, 2023, at 11:23 AM, LEONARDO MUTTI <leonardo.mutti01 at universitadipavia.it> wrote:
> 
> Thank you for the help. 
> Adding to my example:
>       call PCGASMSetSubdomains(pc,NSub, subdomains_IS, inflated_IS,ierr)
>       call PCGASMDestroySubdomains(NSub,subdomains_IS,inflated_IS,ierr)
> results in:
>       Error LNK2019 unresolved external symbol PCGASMDESTROYSUBDOMAINS referenced in function ...  
>       Error LNK2019 unresolved external symbol PCGASMSETSUBDOMAINS referenced in function ...  
> I'm not sure if the interfaces are missing or if I have a compilation problem.
> Thank you again. 
> Best,
> Leonardo
> 
> Il giorno sab 29 apr 2023 alle ore 20:30 Barry Smith <bsmith at petsc.dev <mailto:bsmith at petsc.dev>> ha scritto:
>> 
>>    Thank you for the test code. I have a fix in the branch  barry/2023-04-29/fix-pcasmcreatesubdomains2d <https://gitlab.com/petsc/petsc/-/commits/barry/2023-04-29/fix-pcasmcreatesubdomains2d> with merge request https://gitlab.com/petsc/petsc/-/merge_requests/6394
>> 
>>    The functions did not have proper Fortran stubs and interfaces so I had to provide them manually in the new branch.
>> 
>>    Use
>> 
>>    git fetch
>>    git checkout barry/2023-04-29/fix-pcasmcreatesubdomains2d <https://gitlab.com/petsc/petsc/-/commits/barry/2023-04-29/fix-pcasmcreatesubdomains2d>
>>    ./configure etc
>> 
>>    Your now working test code is in src/ksp/ksp/tests/ex71f.F90  I had to change things slightly and I updated the error handling for the latest version.
>> 
>>    Please let us know if you have any later questions.
>> 
>>   Barry
>> 
>> 
>> 
>> 
>>> On Apr 28, 2023, at 12:07 PM, LEONARDO MUTTI <leonardo.mutti01 at universitadipavia.it <mailto:leonardo.mutti01 at universitadipavia.it>> wrote:
>>> 
>>> Hello. I am having a hard time understanding the index sets to feed PCGASMSetSubdomains, and I am working in Fortran (as a PETSc novice). To get more intuition on how the IS objects behave I tried the following minimal (non) working example, which should tile a 16x16 matrix into 16 square, non-overlapping submatrices:
>>> 
>>> #include <petsc/finclude/petscmat.h>
>>> #include <petsc/finclude/petscksp.h>
>>> #include <petsc/finclude/petscpc.h>
>>>       USE petscmat
>>>       USE petscksp
>>>       USE petscpc
>>>       
>>>       Mat   :: A
>>>       PetscInt :: M, NSubx, dof, overlap, NSub
>>>       INTEGER :: I,J
>>>       PetscErrorCode      :: ierr
>>>       PetscScalar :: v
>>>       KSP            :: ksp
>>>       PC             :: pc
>>>       IS :: subdomains_IS, inflated_IS
>>>       
>>>       call PetscInitialize(PETSC_NULL_CHARACTER , ierr)
>>>       
>>> !-----Create a dummy matrix
>>>       M = 16
>>>       call MatCreateAIJ(MPI_COMM_WORLD, PETSC_DECIDE, PETSC_DECIDE,
>>>      & M, M,
>>>      & PETSC_DEFAULT_INTEGER, PETSC_NULL_INTEGER,
>>>      & PETSC_DEFAULT_INTEGER, PETSC_NULL_INTEGER,
>>>      & A, ierr)
>>>       
>>>       DO I=1,M
>>>          DO J=1,M
>>>             v = I*J
>>>             CALL MatSetValue (A,I-1,J-1,v,
>>>      &                     INSERT_VALUES , ierr)
>>>          END DO
>>>       END DO
>>>       
>>>       call MatAssemblyBegin(A,MAT_FINAL_ASSEMBLY , ierr)
>>>       call MatAssemblyEnd(A,MAT_FINAL_ASSEMBLY , ierr)
>>>       
>>> !-----Create KSP and PC
>>>       call KSPCreate(PETSC_COMM_WORLD,ksp, ierr)
>>>       call KSPSetOperators(ksp,A,A, ierr)
>>>       call KSPSetType(ksp,"bcgs",ierr)
>>>       call KSPGetPC(ksp,pc,ierr)
>>>       call KSPSetUp(ksp, ierr)
>>>       call PCSetType(pc,PCGASM, ierr)
>>>       call PCSetUp(pc , ierr)
>>>       
>>> !-----GASM setup
>>>       NSubx = 4
>>>       dof = 1
>>>       overlap = 0
>>>       
>>>       call PCGASMCreateSubdomains2D(pc, 
>>>      &      M, M,
>>>      &      NSubx, NSubx,
>>>      &      dof, overlap,
>>>      &      NSub, subdomains_IS, inflated_IS, ierr)
>>> 
>>>       call ISView(subdomains_IS, PETSC_VIEWER_STDOUT_WORLD, ierr)
>>>       
>>>       call KSPDestroy(ksp, ierr)  
>>>       call PetscFinalize(ierr)
>>> 
>>> Running this on one processor, I get NSub = 4.
>>> If PCASM and PCASMCreateSubdomains2D are used instead, I get NSub = 16 as expected.
>>> Moreover, I get in the end "forrtl: severe (157): Program Exception - access violation". So:
>>> 1) why do I get two different results with ASM, and GASM?
>>> 2) why do I get access violation and how can I solve this?
>>> In fact, in C, subdomains_IS, inflated_IS should pointers to IS objects. As I see on the Fortran interface, the arguments to PCGASMCreateSubdomains2D are IS objects:
>>> 
>>>        subroutine PCGASMCreateSubdomains2D(a,b,c,d,e,f,g,h,i,j,z)
>>>        import tPC,tIS
>>>        PC a ! PC
>>>        PetscInt b ! PetscInt
>>>        PetscInt c ! PetscInt
>>>        PetscInt d ! PetscInt
>>>        PetscInt e ! PetscInt
>>>        PetscInt f ! PetscInt
>>>        PetscInt g ! PetscInt
>>>        PetscInt h ! PetscInt
>>>        IS i ! IS
>>>        IS j ! IS
>>>        PetscErrorCode z
>>>        end subroutine PCGASMCreateSubdomains2D
>>> Thus:
>>> 3) what should be inside e.g., subdomains_IS? I expect it to contain, for every created subdomain, the list of rows and columns defining the subblock in the matrix, am I right?
>>> 
>>> Context: I have a block-tridiagonal system arising from space-time finite elements, and I want to solve it with GMRES+PCGASM preconditioner, where each overlapping submatrix is on the diagonal and of size 3x3 blocks (and spanning multiple processes). This is PETSc 3.17.1 on Windows.
>>> 
>>> Thanks in advance,
>>> Leonardo
>> 

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